Glacial lake outburst megafloods (discharge ≥106 m3/s) provide natural experiments for understanding how catastrophic events shape landscapes across geologic timescales. While most megaflood research focuses on low-relief systems, their behavior in mountainous terrain remains poorly understood. The Yarlung-Siang River in the eastern Himalaya offers an ideal case study, with evidence of repeated Quaternary megafloods sourced from glacially-dammed lakes on the Tibetan Plateau. Using 2D hydraulic simulations of an 81 km3 paleolake outburst, this work demonstrates that valley morphology fundamentally controls megaflood hydraulics, creating erosional patterns that differ dramatically from modern stream power distributions. These simulations reveal megafloods can deposit boulders >4 meters in diameter throughout the flood pathway—far larger than the modern river can transport. Field observations confirm 106 boulder bars containing boulders >4 m in diameter. To understanding the long-term landscape impacts of these massive, immobile boulders, we can use a 1D landscape evolution model to incorporate boulder effects on channel erosion. Results show megaflood boulder bars generate meter-scale knickpoints persisting >20 kyr, fundamentally altering local channel steepness. With repeated megaflood during glacial periods, these effects compound, creating stepped longitudinal profiles. This work demonstrates that megaflood legacies extend beyond erosion to include persistent depositional effects influencing landscape evolution for millennia.

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